I may be (and am quite likely to be) completely wrong but I do think that what they need more than anything is to be listened to and empathised with, to feel that you're "on their side". I do lots of agreeing that it must be very hard and horrid not to enjoy school and very difficult to be facing so many exams and to be worried about failing etc...
I can vividly remember my own mum basically refusing to hear anything less than positive (NOT saying that's what you are doing Stropps!) saying, "of course you don't hate school, don't be so ridiculous" etc and it made me feel very alone and misunderstood, as well as absolutely fucking furious.
They need you to be able to withstand their own anxieties and fears without joining in. Oh, and don't underestimate the importance of staying very calm and providing lots of food and clean laundry...
Please don't for a moment think I've got all this teen business sorted because I soooo haven't. But that is what I feel I've learnt so far seeing Ds1 through GCSEs last year.